With the development of the mobile communication systems, communication systems of various radio access technologies (RATs) are emerging, and networks suitable for such communication systems are being established. Such a network infrastructure is installed independently by a number of service providers. Thus, as shown in FIG. 2, base stations of different RATs overlap with each other at the same point.
Moreover, these communication systems each have different characteristics. Thus, while there are systems that provide low mobility but high bandwidth, there are also systems that provide low bandwidth but high mobility and wide coverage.
To match with these communication systems, research on multimode terminals for supporting communication systems of different RAT in one terminal is underway in terms of terminals, and, recently, terminals supporting multi-modes are being launched into the market.
In this circumstance, there is a need for a user to decide an optimum communication system among those communications system of different RATs to connect and get service. Moreover, to make such a decision, it is necessary for a terminal to recognize a base station accessible from the current location. If no accessible base station is found, all interfaces have to be operated to measure signals and find an accessible system, which requires long time and large power consumption.
Therefore, studies on the technology to minimize power consumption and fast search for an accessible base station have been conducted. Such a technology is called a base station search, which is divided into three depending on its methods.
The first is a method in which a terminal transmits information required for a base station search in a network, which is called a centralized method. In this method, the terminal determines a base station to be accessed based on the information provided from the network. To this end, a server for storing and managing base station location information of each communication system is installed in the network. Also, there is a need for measures to transmit this information to the terminal.
The second is a method in which a terminal measures radio signals and finds an accessible base station without prior information, which is called a distributed method. In this method, a wireless interface of a communication network expected to be accessible by a terminal is switched on to receive pilots or beacons of the communication system and then the presence or absence of the communication system is determined based on the received pilots or beacons.
The third is a combination of the above two methods, which is called a hybrid method. In this method, a network provides base station location information to a terminal so that the terminal performs a search for a base station of an actual communication system based on the base station location information. Measures to provide network base station information to enable a terminal to perform a base station search have been suggested to have the same concept although the functions and names are slightly different depending on research organizations and standard-setting organizations. CPC (Cognitive Pilot Channel), which is proposed in the E2R project in Europe, is also within this concept.
Hereinafter, a process in which a mobile terminal searches for a base station by a paging procedure when a paging area changes will be described with reference to the accompanying drawings.
FIG. 1 is a flowchart showing a conventional paging procedure. First, when a paging area changes, a mobile node (MN) is connected to a paging agent (PA) via a radio access network (RAN) serving as an access network to perform a location registration procedure. Then, the paging agent receives subscriber information of the mobile node from a home location register (HLR) to check whether the mobile node can get a location registration service at the current location by using the subscriber information.
Upon completion of check on the mobile by using the subscriber information, the paging agent completes the location registration procedure. Then, upon receipt of a packet from a counterpart node, the paging agent sends a paging request message to all RANs within a paging area where the location of the mobile node is registered. The paging request message contains quality of service (QoS) of a data packet having issued the paging and the ID of mobile node. Among mobile nodes that have received the paging request message, a mobile node whose ID matches the ID of the mobile node within the paging request message establishes a wireless channel with the paging agent. Then, the mobile node sends a paging response message for the paging request message to the paging agent, and the remaining mobile nodes are transited to an idle state.
The paging agent, which has received the paging response message, establishes a data channel with the mobile node depending on the QoS information of the packet, so that the mobile node performs communication with the counter node.
The conventional base station search method has the drawback of high power consumption since it has to be carried out periodically in the idle state without being taken into account in the paging step.
Moreover, in the conventional base station search method, in order for a mobile node to search for an optimum base station, a base station search has to be performed periodically all the time to get information of a base station accessible from the current location. To this end, the base station search has to be performed constantly even in the idle state, resulting in increased power consumption.